This invention relates to silicon controlled rectifiers and, more particularly, to gold doped fast turnoff silicon controlled rectifiers.
Since their introduction, SCR's have become more popular as circuit designers have continued to employ them in new and different ways. As new applications for SCR's are developed, greater emphasis is placed on improving the performance of the devices under demanding conditions of operation. Different applications require that the emphasis be placed on different characteristics of the device. For example, some potential applications will become commercially feasible only if the cost of the SCR's can be reduced. Other applications require that the devices be able to withstand higher reverse voltages or that they switch from a conductive to a non-conductive state more rapidly. Thus, the device designer is faced with a multi-faceted problem.
One of the principle problems facing the device designer is the interrelationship among the various device characteristics. For example, it is known that if gold is diffused into the interior n doped region of an SCR, the minority carrier lifetime in that region is decreased and thus the turn off time is reduced. However, the effect of the gold diffusion step on the cost of the device must be considered. Also, it is known that glass passivation of the semiconductor pellets forming the SCR's has many beneficial effects. For example, the reverse voltage characteristics are improved as is device reliability. Furthermore, glass passivation improves the manufacturing yield and thus reduces cost. Consequently, an SCR exhibiting desirable properties could seemingly be made by utilizing both gold diffusion and glass passivation techniques. However, this has not heretofore been possible. This is so for the following reason. Glass melts at a lower temperature than that required to perform the gold diffusion. Thus, the glass passivation step must be performed following the gold diffusion step. But the glass passivation process includes an oxide growing step which is performed at a high enough temperature to cause uncontrollable migration and redistribution of the gold. Thus, heretofore, selectively localized gold doping and glass passivation have been considered incompatible operations. Consequently, gold doped SCR's are generally manufactured by a process including blanket gold diffusion of an entire wafer followed by pelletization and individual passivation with silicone rubber.
It is an object of this invention, therefore, to provide an SCR with gold doping and glass passivation and to disclose a method for making such a device.